Three-prong plug



May 30, 1961 w. c. BENDER, JR

THREE-PRONG PLUG Filed July 30, 1958 INVENTOR: WILLIAM C. BENDER, JR.

BY v

ATT'YS United States, Patent THREE-PRONG PLUG William C. Bender, J12, 522 Mitchell, Elmhurst, Ill.

' Filed July 30, 1958, Ser. No, 151,979

2 Claims. (Cl. 339-14) This invention relates to the electric plugs for use with receptacles for either two-wire or three-wire circuits.

The Underwriters Laboratories, Inc. and many electric codes long have required that all portable electrical tools be equipped with a connector for grounding the plug when inserted into a receptacle to supply current for the operation of the tool. In the past this requirement has been complied with by the use of what is commonly called a pigtail connection in the form of a wire coming out of one side of the plug and. one of the blades of the plug with a contact for attachment to a neutral ground lead, such as the box carrying the receptacle.

Currently the Underwriters are insisting that plugs for these portable tools must be equipped with a fixed prong for insertion into three-wire receptacles. At the present time most buildings, where portable electric tools are likely to be used, have only two-wire receptacles. That presents a problem to the manufacturers of portable electric tools.

The main objects of this invention are to provide an improved form of three-prong electric plug suitable for use with either two-wire or three-wire receptacles; to provide an improved three-prong plug which permits a quick shifting of the third prong between use and nonuse positions; and to provide an improved three-prong plug of this kind which is so simple in construction as to make its manufacture very economical and its use on the job extremely facile.

One embodiment of this invention is shown in the accompanying drawings, in which:

Figure 1 is an enlarged, side view, partly broken away, of an improved three-prong plug constructed in accordance with this invention;

Fig. 2 is an end view of the same, taken from the left of Fig. 1;

Fig. 3 is a longitudinal sectional view taken on the plane of the line 3-3 of Fig. 2; and

Fig. 4 is a perspective view of the third prong and its mounting removed from the support.

The essential concept of this invention involves a conventional molded rubber support with a supplemental prong hinged to a channel-shaped, spring-metal mounting so embedded in the support as to permit the hinged prong to be shifted between oppositely-extending positions parallel with the conventional contact blades of the plug.

A three-prong plug embodying the foregoing concept comprises a support or body 5 of rubber or other nonelectrical conducting material wherein are fixed a pair of conventional conductor terminal blades 6 and 7 and wherein is formed a recess embedding a channelshaped mounting 8 to which is hinged a prong ground blade 9 and swingable 180 degrees, through a slit 10a in the support 5, between oppositely-disposed positions substantially parallel with the conductor terminal blades 6 and 7, and into the cylindrical recess 10b. In the position shown in Fig. 3 the portion of the blade 9 adjacent the recess 10, seats in a recess 100.

The support 5 is molded rubber such as currently is used extensively for terminal plugs. support 5 is of a somewhat frusto-conical form of pentagonal cross-section. (See Fig. 2.) The sides 11 and 5 12, leading from the nearly but not quite parallel sides 13 and 14, converge to form a ridge section 15 opposed to the slightly-curved, fifth or base side 16.

It should be understood that such a form of the support 5 is not critical to this invention. Rather, it is 10 primarily a conveniently-producible and easily-handled shape.

As is well-known in molded plugs of this kind, the

terminal blades 6 and 7 are flat pieces of metal with clips 17 integrated with one end. These clips are bent 15 to clamp the uncovered ends of wires 18 leading from the end 19 of the support 5. The opposite ends of the blades 6 and 7 extend outwardly parallel from the flat face 20 of the support 5.

As most clearly shown in Fig. 2, these two conductor 20 blades 6 and 7 are arranged to one side of the longitudinal median of the support 5 extending transversely of the blades 6 and 7. This is done to afford ample room for embedding the mounting 8.

The mounting 8 is a strip of spring metal formed U-shape with the lateral edges of the sides 21 flared outwardly to form opposed flanges 22. The portions of the sides 21, between the flanges 22 and the base 23, are oppositely curved to form concave seats for the prong 9. At one end of the mounting 8 is an integral clip 24,

similar to those on the blades 6 and 7, whereto is attached a third wire 25 extending out through the support end 19 with the conductor wires 18.

The blade 9 is a section of round, heavy-gauge wire hinged to the mounting 8 by a pin 26 medially located on the sides 21 of the mounting 5 and extending through the blade 9 adjacent one end. The pin 26 thus positions the prong to permit its swinging through 180 degrees to be disposed in opposite directions in substantially the same plane and practically parallel with the blades 6 and 7.

The mounting 8 is embedded in the recess 10 in the ridge section 15 of the support 5 in parallel opposition to the blades 6 and 7 so as to locate the blade 9 for swinging through the slit 10a. When the blade 9 is swung into the position as shown in full lines in Fig. l and 3 it extends outwardly from the face 20 of the support 5 parallel with the blades 6 and 7. Thus, when the plug is inserted into a three-wire receptacle (not shown here) the prong 9 will enter a conventional receptacle socket at the same time that the blades 6 and 7 enter the rectangular-shaped sockets. When the plug is to be used with a two-wire receptacle, the blade 9 is swung rearwardly 180 degrees back into the recess 10b as shown in dotted outline in Fig. 3. So positioned the prong 9 is nearly concealed and is completely out of 55 the way.

When desired, this improved three-prong plug may have secured thereto the commonly-used, supplemental ground wire or pigtail 27. Such a wire 27 has a connector 28 on one end and the opposite end is secured to the clip 24 on the mounting 8, as most clearly shown in Fig. 3.

Although the ground prong or blade 9 is shown herein as being cylindrical it will be apparent that it may take on the conventional flat elongated shape such as blades Patented May, 30, 1961 Q As here shown, the v to the surface of said housing and embracing said mounting, a ground contact blade pivotally secured to said mounting, said ground contact blade being manually swingable on its pivot through the housing cleft in a plane normal to the common plane of said pair of contacts into and out of operative positions disposed parallel to said contacts with the extremity of the free end of the blade exposed beyond the end of the housing when the blade is in its inoperative position to permit finger contact with the blade to effect its swinging into its operative position.

2. An electric plug comprising, a molded support tapered from a flat end and having a longitudinal recess formed therein along one side of the longitudinal median and communicating with a slit opening to the periphery of the support, a pair of non-movable parallel contact blades embedded in the support on the opposite side of the longitudinal median from the recess, the blades extending outwardly from the flat face of the support, conductor wires connected to the blades and extending outwardly from the tapered end of the support, a channel-shaped spring-metal mounting embedded in the recess inwardly of the slit and intermediate the ends of the support with the longitudinal axis of the mounting disposed parallel with the longitudinal axis of the support, a prong hinged adjacent one end to the mounting intermediate its ends for swinging through the slit substantially degrees between a position extending outwardly from the flat face of the support parallel with the blades and oppositely-extending retracted position substantially concealed within the recess and parallel with said pair of blades, but vw'th the extermity of the free end of the prong exposed beyond the reduced end of the housing to permit finger contact with the prong to effect its swinging into its operative position and a conductor wire connected to the prong and extending outwardly of the support.

References Cited in the file of this patent UNITED STATES PATENTS 1,026,653 Brunt May 21, 1912 2,182,446 Nelson Dec. 5, 1939 2,922,134 Elfenbein Jan. 19, 1960 FOREIGN PATENTS 243,210 Switzerland June 30, 1946 326,324 Switzerland Apr. 15, 1958 684,915 Great Britain Dec. 24, 1952 794,339 France Dec. 12, 1935 OTHER REFERENCES Inskip et al.: Electrical Eng, April 1955, page 289. 

